Chlorine dioxide (ClO2) is used in the preparation of service water, for example of drinking water, or in swimming pools for disinfection, i.e., for killing pathogens. The chlorine dioxide concentration is monitored and controlled or regulated in order to dose the chlorine dioxide used, and/or to monitor the water quality. For example, sensors that operate according to the amperometric measurement principle may be used for this purpose.
Amperometric sensors that are available today for determining a chlorine dioxide concentration or a value correlated therewith, for example a chlorine dioxide activity or a chlorine dioxide partial pressure, normally comprise a measurement probe that can be immersed into a measuring fluid, which measurement probe can be connected with superordinate sensor electronics, for example a measuring transducer or other electronics for measurement value processing. The measurement probe of these sensors comprises a probe housing in which is formed a housing chamber sealed by a membrane and filled with an inner electrolyte. The membrane is arranged within a region of the measurement probe designated for contact with the measuring fluid and is permeable, in particular selectively permeable to chlorine dioxide, such that chlorine dioxide may get from the measuring fluid into the housing chamber and vice versa.
At least two electrodes contacting the inner electrolyte are arranged within the housing chamber. In some embodiments, three electrodes may also be present. The electrodes are connected in an electrically conductive manner with the measurement circuit arranged outside of the housing chamber, which measurement circuit may be a component of on-site electronics arranged in the probe housing, for example. One of the electrodes is used as working electrode, another as counter electrode. To determine the measurand, a predetermined voltage is applied by means of the measurement circuit between the working electrode and the counter electrode, wherein the working electrode is connected as a cathode and the counter electrode is connected as an anode, so that chlorine dioxide contained in the inner electrolyte is electrochemically converted at the cathode. The current flowing through the inner electrolyte between the working electrode and the counter electrode is detected by means of the measurement circuit as a measure of the chlorine dioxide content in the measuring fluid. Given a measuring gas, the chlorine dioxide content may be indicated as a partial pressure; given a measuring liquid, the chlorine dioxide content may be indicated as a concentration. In applications with three electrodes, the potential of the working electrode or the current flow through the working electrode may be regulated by means of a third reference electrode through which current does not flow.
The inner electrolyte of these commercially available sensors is typically in the acidic or neutral pH range. However, it has surprisingly been shown that these conventional chlorine dioxide sensors can deliver incorrect measurement results given use in acidic measuring fluids.
From U.S. Pat. No. 4,176,032, a chlorine dioxide sensor is known that has a working electrode made of gold or platinum and a counter electrode made of silver or copper. In the absence of chlorine dioxide, the working electrode is negatively polarized and the counter electrode is positively charged, whereas in the presence of chlorine dioxide in the inner electrolyte, a reaction occurs at the working electrode, which reaction partially depolarizes the working electrode so that the working electrode may take up released electrons given an oxidation taking place at the counter electrode. This leads to a current flow that serves as a measurement signal and is linearly dependent on the chlorine dioxide concentration in the inner electrolyte. The inner electrolyte is an aqueous solution of a halide salt that additionally contains a complexing agent, for example EDTA, which complexes copper or silver ions that are formed in the oxidation reaction occurring at the counter electrode. The inner electrolyte may additionally contain a pH buffer. The complexing of the copper or silver ions passing into solution prevents a passivating copper oxide or silver oxide layer from forming at the counter electrode. Given that a sufficient concentration of the complexing agent in the inner electrolyte must be ensured in this sensor, the selection of the pH value of the inner electrolyte is possibly limited. U.S. Pat. No. 4,176,032 specifies EDTA as a suitable complexing agent. However, EDTA has a sufficient solubility only at high pH values, meaning in the basic pH range. Given low pH values, the EDTA concentration that is present in solution in the inner electrolyte may no longer be sufficient to prevent a negative effect on the sensor function. Complexing agents suitable for low pH values are not specified in U.S. Pat. No. 4,176,032.